CH not switching off - Nest/Viessmann

Hello

I have a NEST with Viessmann system boiler with unvented cylinder.
I have a mid position and two position valve




I thought the mid position valve was faulty as it was not switching off
I got new actuator - the right one (40003916-003/U) . The manuals do make a reference to v4073a1039 so I'm sure it is the right one.

However my CH does not turn off when I switch it off in the app (when there is no HW on).
If HW and CH are on and I turn CH off it does work

I think my NEST is connected ok

App status Nest wiring status Mid posn valve wires

Both off 1245 live Grey live

Ch on 2345 White/Grey Live

HW on 1256 Neither live

Both on 2356 White live

I also notice some spurious voltages for eg after I turn Ch off 3 on nest has 55v (white wire as well)
When both CH and HW are on 4 on nest has 72v (also Grey wire)


Any clues on what is going wrong here

The spring does bring lever back to mid position when there is no power (I switch mains off) so i doubt the spindle is getting stuck.

When I turn power on with CH/HW off on app and force the lever all the way to left (feel spring resistance) the boiler comes on and valve remains open (loose lever)